Recently, there has been an increasing demand of electrochemical devices with high-capacity and high-output in the field of small, medium and large electrochemical devices. The high-capacity and high-output electrochemical devices generally have a separator in the form of a thin film, and such a separator is required to have low electrical resistance and maintain safety.
In the case of the high-capacity and high-output electrochemical device, the thickness of a separator decreases as loading amounts of electrodes increase within limited spaces, and this electrochemical device needs a separator having high porosity for its high output. However, the high porous separator has very poor mechanical and thermal properties. Particularly, the separator fails to suppress thermal propagation during safety tests (impact, nail penetration, hot box and so on), thereby causing ignition and explosion on heating. For this reason, in spite of electrochemical devices having good performances, the electrochemical devices often fail to get certification and are not used.
In the separator, three types of substrates according to preparation methods may be used. One of the preparation methods is to make polyolefins into thin fiber to obtain a porous substrate in the form of a non-woven fabric. Another is a drying method that stretches a thick polyolefin film at a low temperature to cause micro cracks between lamella being crystalline structure of the polyolefin film, thereby forming micropores. The other is a wetting method that mixes polyolefins with diluents to form a single phase, brings the polyolefins and the diluents into phase-separation during cooling, and extracts the part of the diluents to form pores in the polyolefins.
The separator consisting of only such a substrate has poor thermal/mechanical properties. In order to enhance safety, the porous substrate has been provided with a heat-resistant coating of organic particles/inorganic particles thereon to prepare a composite separator, but there is a still need of improvement.